1. Field of the Invention
The present invention relates to a substrate cutting method and particularly it relates to a substrate cutting method suitable for cutting a semiconductor wafer having a number of elements formed therein to produce semiconductor pellets.
2. Brief Description of the Prior Art
Semiconductor devices are produced, generally, by forming a number of elements (including integrated circuits; hereinafter the same) in a semiconductor wafer sliced from an ingot and polished, cutting said semiconductor wafer along scribed lines between said elements to provide pellets, die-bonding said pellets to a lead frame or the like, making wire-bonding between the elements and the leads, molding them in resin or the like, and dividing said lead frame or the like.
The cutting of said semiconductor wafer is effected, as shown in FIGS. 10 and 11, by sticking a wafer W formed with a number of elements P to an adhesive sheet S, holding the adhesive sheet S by suction through suction holes H in an x-y table ST, dicing the wafer along scribed lines between the elements P by a dicer D having a diamond blade B, thereby dividing the individual elements P to provide pellets P.
However, the method of cutting the wafer W using the dicer D by sticking it to the adhesive sheet S has the following problems.
First, cutting by mechanically forming and proliferating material defects by a dicer D results in cracks or chippings produced in the wafer W or elements P during cutting, thus decreasing the yield of the pellets P. Although apparent cracks or chippings can be detected and removed by imaging as by a camera, microcracks or the like produced in the interior are difficult to detect by external imaging, leading to defectives found as by characteristic examination after assembly, thus resulting in wasting not only adhesive agents, heat dissipation plates, etc., during die-bonding, and wires and other materials during wire-bonding but also time, electricity, gas, etc., which result from applying unnecessary process treatments.
Second, cutting by the dicer D inevitably involves cooling since heat of friction is produced. Further, cuttings are produced by dicing and to wash away the cuttings, a large amount of cooling water is required during dicing. Further, because of the cooling water, the device has to be made water-tight and hence the device becomes complicated and expensive.
Third, recently, there has been a growing demand for thin type semiconductor devices, such as solar cells, IC cards, and stack type semiconductor devices. However, thinning the wafer W to cope with such demand for thinning lowers the mechanical strength, so that the wafer W tends to be damaged by the pressing force with which the wafer W is stuck to the adhesive sheet S and moreover the pellets P tend to be damaged when peeled from the adhesive sheet S after cutting.
Therefore, for thinning the pellets P, a production method, referred to as foredicing, as shown in FIGS. 12(A)-(D) has been developed. This method comprises the steps of (A) forming a number of elements P on the front side a of a relatively thick wafer W having a thickness t1 (for example, 500 xcexcm), and sticking the back b to a first adhesive sheet S1, (B) dicing the wafer W from the front side a along scribed lines between elements P to form a grove G of predetermined depth, (C) peeling the adhesive sheet S on the back side, and sticking a second adhesive sheet S2 this time to the front a side, and (D) grinding the back b side to remove the material by an amount corresponding to a thickness t3 which exceeds the groove G formed by said dicing, thereby forming a thinned back c and at the same time dividing into individual pellets P, thus providing pellets P of desired thickness t2 (for example, 30-50 xcexcm)
However, this production method referred to as foredicing is troublesome and greatly increases production costs. Further, when the pellets P are peeled from the adhesive sheet S2, the fact remains that the pellets P tend to be damaged.
Fourth, as shown in FIGS. 13(A)-(D), there has been developed another production method based on a batch adhesive agent layer, comprising the steps of (A) forming an adhesive agent layer AD of solder, resin or the like on the back of the wafer W in advance by batch-processing and sticking the adhesive agent layer AD side to the adhesive sheet S, (B) cutting it into pellets P by dicing, (C) peeling the pellets P from the adhesive sheet S to provide pellets P having the adhesive agent layer AD on the back, and (D) die-bonding the pellets P to a heat dissipation plate R, such as a lead frame, by utilizing the adhesive agent layer AD on the back.
Since this method is not required to feed an adhesive agent to the heat dissipation plate R meticulously during die-bonding, the die-bonding process becomes easier and can be reduced in time. Moreover, the adhesive agent layers of the pellets P bonded to the heat dissipation plate R become uniform in thickness, the height of the bonding position becomes constant in the subsequent wire bonding process. Therefore, the troublesome height adjustment of the bonding tool for each bonding place becomes unnecessary; thus, the production method allows quick, easy and reliable wire bonding. With the production method for thinning the pellets P, referred to as foredicing, described above, however, pellets P having the adhesive agent layer AD on the back cannot be obtained by forming the adhesive agent layer AD of solder, resin or the like on the back of the wafer W in advance. Further, in the case where the adhesive agent layer AD is formed of soft material, such as solder, the adhesive agent clogs the blade B, making the satisfactory dicing impossible.
Fifth, the method using a dicer D comprises the steps of setting the height of the blade B at a position deviated from the wafer W position, horizontally moving an x-y table with the wafer W held thereon by drawing while maintaining the blade height, thereby reciprocating the blade from a position outwardly of one end of the wafer W to a position outwardly of the other end and vice versa to dice the wafer W; therefore, non-square pellets are produced in large numbers around the periphery of the wafer W, making it necessary to sort out square pellets from non-square pellets, which is a troublesome process.
Thus, a method may be contemplated which, rather than mechanically cutting the wafer W by the dicer D having a diamond blade B, comprises the steps, as shown in FIGS. 14(A)-(C), of (A) irradiating CO2 laser, YAG laser or other continuous wave laser, or long pulse laser L along scribed lines between the pellets P of a wafer W, (B) melting and scattering a laser-irradiated portion to form a groove G, and (C) repeatedly irradiating laser L to grow the groove G through the back so as to cut the wafer W.
However, since such laser L a continuous wave or has a large pulse width, laser irradiation results in a temperature rise in the vicinity of the laser-irradiated portion by heat conduction, producing a heat strain in the wafer W, which not only forms a cause of cracks or microcracks but also heats and melts up to the vicinity of the portion irradiated with the laser L. And since the melted portion is scattered together by the drastic scattering force of the melt of the portion irradiated with laser L, the width w of the groove G being formed becomes large and nonuniform. Further, since the angle of inclination of the groove G is small, it is necessary to set the scribed line width between the elements P at a large value, resulting in a decrease in the pellet yield. Moreover, the melted wafer material deposits on the edge of the groove G or scatters in the vicinity of the laser-irradiated portion to stick to the electrodes or the like of the elements (pellets) P. Further, since the angle of inclination of the lateral end surface Pa of the pellet P obtained is small, there has been a problem that it sometimes constitutes obstacle depending on the use of the pellets P.
Accordingly, an object of the invention is to provide a substrate cutting method using laser irradiation, wherein the prior art problems described above are solved.
A substrate cutting method of the invention is characterized by irradiating a substrate with ultrashort pulse laser to cut it.
FIG. 4 is a block diagram of an ultrashort pulse laser device. Trying to directly amplify titanium sapphire laser output would result in the peak strength being too high, damaging the optical elements; therefore, a chirped pulse amplification method is used. The chirped pulse amplification method refers to a technique comprising the steps, as shown in FIG. 5, of (1) causing the pulse width of ultrashort pulse laser TL incident on a regenerative amplifier RA to chirp by using a diffraction grating pair, thereby (1) increasing the pulse width more than thousands of times (pulse stretching), (2) amplifying while keeping the peak power low (pulse amplification), and then (3) compressing to the original pulse width by the diffraction grating pair (pulse compression). The finally amplified pulse, for example, has an energy of 2 mJ, a pulse width of 130 fs, and a repetition rate of 10 Hz, and the peak strength is amplified to 15 GW. Since the peak strength of titanium sapphire laser is 107 kW, it follows that it has been amplified about 100,000 times.
The substrate cutting method which irradiates ultrashort pulse laser, unlike mechanical cutting using a dicer having a diamond blade, has the possibility of pellets being scattered by mechanical force during cutting, so that the conventional adhesive sheet is unnecessary and can be dispensed with. Therefore, material costs can be saved and moreover since the sticking process can be reduced, process costs are low. Further, because of dry process, the cleaning process can be omitted. Further, ultrashort pulse laser, as compared with the conventional method of cutting the wafer W by irradiating CO2 laser, YAG laser or other continuous wave laser or pulse laser L, has a short laser pulse width, so that the heat conduction is low; therefore, and there is almost no rise in the substrate temperature in the vicinity of the laser-irradiated portion; thus precluding a decrease in yield due to cracking caused by thermal strain produced by a rise in the temperature of the substrate. Since a narrow groove can be formed only in the portion irradiated with laser, the scribed line width can be designed to be narrow, thus increasing the number of elements per substrate. Moreover, since the possibility of the melted substrate material scattering in the vicinity of the laser-irradiated portion is decreased, the yield of pellets can be increased.
That is, the thermal diffusion length LD during laser irradiation is LD=(Dxcfx841)1/2 where D is the diffusion coefficient of the material and xcfx841 is the pulse width of the laser. Here, D=kT/xcfx81cp where kT, xcfx81, and cp are heat conductivity, density and heat capacity, respectively. Thus, since the heat diffusion length LD is proportional to the square root of the pulse width xcfx841, ultrashort pulse laser irradiation results in the heat diffusion length being greatly reduced as compared with the prior art during laser irradiation, and when the pulse width becomes less than picoseconds, heat diffusion can be almost entirely neglected.
A substrate cutting method of the invention is characterized in that the pulse width of said ultrashort pulse laser is less than 1 picosecond.
According to the substrate cutting method of the invention, the pulse width is less than 1 picosecond. For example, irradiation with femtosecond pulse from a titanium sapphire laser source results in almost no rise in temperature being found in the vicinity of the portion irradiated with laser 7 when the substrate 1 is irradiated with the laser 7, as shown in FIG. 6(A). Therefore, there is formed a groove 8 having steep lateral surfaces only in the portion irradiated with the laser 7, as shown in FIG. 6(B). Irradiation with the laser 7 with a repletion frequency of 1 kHz-100 kHz results in pellets 2 being obtained with the lateral end surfaces 2a having almost no slope, as shown in FIG. 6(C). Therefore, the aspect ratio of the groove 8 is high, and the scribed line width of the substrate 1 can be set at a small value, making it possible to increase the number of pellets 2 per substrate and to improve the yield of the pellets 2. When this is compared with FIGS. 14(A)-(C) described above showing the conventional substrate cutting method using laser, the differences are obvious. Further, there is almost no rise in the temperature of the substrate, nor is the deposition or scatter of melted substrate material in the vicinity of the laser-irradiated portion.
A substrate cutting method of the invention is characterized in that the surface layer of the substrate which is in an improved state is irradiated with said laser.
According to the substrate cutting method of the invention, laser irradiation is effected in an improved state in which the temperature of the surface layer of the substrate is raised to lower the laser penetration; therefore, the laser absorption coefficient of the improved surface layer becomes high, making it possible to improve the processing accuracy.
The invention of the invention is characterized in that the ultra short pulse laser emits a plurality of pulses having an interpulse separation of 3 to 30 picoseconds.
In the method for cutting a substrate of the invention, scattered particles produced by a preceding pulsed laser beam are prevented from re-adhering to the periphery of the hole which may be caused by the subsequent pulsed laser beam. It is thus made possible to reduce the height of a swelling around the hole.
A substrate cutting method of the invention is characterized in that said substrate is a semiconductor wafer formed with a number of elements and said ultrashort pulse laser is irradiated along the scribed lines between said elements.
According to the substrate cutting method of the invention, since the scribed line width between elements can be reduced as described above, the number of pellets per semiconductor wafer can be increased, and moreover since no cracking or chipping occurs in the pellets, the pellet yield can be greatly improved and so can be the pellet strength.
A substrate cutting method of the invention is characterized in that the thickness of said semiconductor wafer is not more than 50 xcexcm.
According to the substrate cutting method of the invention, a semiconductor device having thinned pellets such as an IC card, now in great demand, or of the stack type can be produced without employing the production method referred to as foredicing.
A substrate cutting method of the invention is characterized in that said semiconductor wafer has a batch-processed adhesive agent layer on the back.
According to the substrate cutting method of the invention, since the pellets obtained by cutting the semiconductor wafer have batch-processed adhesive agent layers on the back, the troublesome operation of meticulously feeding an adhesive agent such as solder or resin to a lead frame or the like during die-bonding to the lead frame or the like can be omitted. Moreover, the presence of the adhesive agent layer of uniform thickness on the back eliminates the possibility of the pellets die-bonded to a lead frame or the like being inclined. In the subsequent wire bonding process, the bonding height position becomes constant, thereby making unnecessary the troublesome operation of adjusting the bonding tool height for each bonding place, thereby not only facilitating the bonding operation but also making the bonding strength in each bonding place constant; thus, a semiconductor device having uniform and superior characteristics is obtained.
A substrate cutting method of the invention is characterized in that the back of said semiconductor wafer is drawn by an x-y table.
According to the substrate cutting method of the invention, since the semiconductor wafer is drawn to the x-y table by static electricity or vacuum suction force during cutting, the aligned state of the elements in the wafer can be retained in the pellets after cutting; therefore, the batch-movement of the plurality of pellets onto a tray or the like after cutting can be easily realized and so can be the successive pick-up of pellets from the x-y table after cutting to die-bond them directly to a lead frame or the like.
A substrate cutting method of the invention is characterized in that said laser is irradiated to the semiconductor wafer excluding the peripheral portion thereof.
According to the substrate cutting method of the invention, since the peripheral portion of the semiconductor wafer is not irradiated with laser, the laser irradiation time can be correspondingly reduced to improve the throughput. Moreover, since the peripheral portion of the wafer is not cut, there is no possibility of a number of non-square pellets due to non-square elements being produced in the peripheral portion of the wafer as in the case of a cutting method using a dicer, and hence the processing of the same is unnecessary, facilitating the production.